At the checkout line you have everything list, your cash or credit card ready — and, oh yeah, get set for one more thing.

"Can I have your phone number, please?"

Privacy advocates advise against revealing such information, because it can be the key to accessing more sensitive information about you.

Because a phone number often can be used to look up a customer's address, stores say it helps them send special offers through the mail or tailor those offers to a customer's prior purchases. Many stores have signs explaining such a policy near checkout counters. Such signs commonly add that the stores don't share their customer information with third parties.

"I think a lot of stores, to be fair, they're not abusing your privacy," said Ponemon, who once audited an unspecified chain store's use of customer data and found it ethical. "But some stores are thinking there's money in your data."